You have nothing to lose by trying any of the online mattresses that offer a free return and full refund. There's no way to know if you'll like a mattress until you sleep on it a few nights, and you can't do that in a store. Most online mattresses will let you try it for 90 or more days and still let you return for a full refund. I recommend trying as many of the online ones as you can.
We tried Casper and Purple. Neither gave us any problems when returning and we got a full refund. If I were making the decision alone, I would have kept the Casper. But my wife didn't like it (I don't remember why). We tried the Purple next and it slept cooler than any mattress I've ever had, and I liked that. But I didn't wake up in the morning feeling like I got a full night's sleep, even though it was comfortable. My wife felt the same way.
We ended up just going to a mattress store and getting one. You may do the same in the end, but I recommend trying out some online ones that offer a trial period with free returns and full refund first. You have nothing to lose.